1. Field of the Invention
The present invention relates to a circuit arrangement for driving gas discharge lamps having a ballast between the positive and the negative output terminals of a rectifier arrangement connected to an AC mains voltage supply through a phase conductor and a neutral conductor, which ballast is equipped with a filter stage, an oscillation voltage build-up circuit, an oscillation inverter having two transistors driven by first and second windings of a current transformer a high-frequency resonant circuit, and also with a decoupling capacitor and a third winding which, having been wound together with the two abovementioned windings of the high-frequency resonant circuit onto a common core, form a current transformer, and which ballast is routed by a connecting line from the junction between the two transistors to one electrode of the discharge lamp.
2. Background and Prior Art
Electrical ballasts for fluorescent lamps have been known for a long time. They are used to raise the mains voltage of usually 110 or 220-250 volts to a substantially higher ignition voltage which may be in the order of magnitude of about 1000 volts. It is also known to raise the frequency of the usual mains frequency of 50 or 60 Hz to a higher frequency which may be from 30 to 40 kHz. These measures have the advantage that the lamp can ignite immediately when turned on.
It is required of the electricity suppliers that, on the one hand, the harmonics do not occur in the radiofrequency range, which would be the case with the third harmonic at 150 kHz if the frequency of the lamp supply voltage were 50 kHz. Consequently, the frequency must be &lt;50 kHz, but for operation it should be &gt;25 kHz. On the other hand, the reactive current is also very problematic, because the measurement and hence the billing are difficult.
A further problem is posed by the peak voltages of the high frequency, because their spikes are super-imposed on the oscillations of the mains voltage. These problems can be solved if, in accordance with DE-A-36 11 611, inductors are connected into the supply line. However, this has the disadvantage that such inductors have to be relatively large at the mains frequency of 50-60 Hz and, consequently, can in no way be built into the lamp stand of a power-saving lamp. In order to reduce the harmonics, DE-A-32 22 534 has proposed a series circuit of an inductor and a diode with two capacitors at the junction point of those two elements. This arrangement leads to a reduction in the harmonics. However, these elements form a high-frequency divider, as a result of which the transistors are heavily loaded. This additional inductor is thus part of the high-frequency resonant circuit and therefore only partially effects interference suppression, with the result that the harmonics are nevertheless higher than permitted, and, moveover, the reactive current component cannot be reduced in this way.